n. 12/2024 79 TECHNOLOGY ché tante e tanto varie possono essere le modalità di impiego. Tale considerazione vale ancora di più per il trattore, che fa della versatilità d’uso il suo punto di forza. Infatti, sono veramente numerose le variabili che possono influire in tal senso: in primis la durata del lavoro effettivo e and the Environmental Protection Agency in the United States, which are based on specific (and presumed) operating cycles of the engines that equip vehicles intended for non-road use. This choice is understandable, since it is the only way possible to objectively establish whether or not a given engine meets the desired requirements. Furthermore, it makes it possible to In the mid-1990s, specific parallel regulations were issued (referred to as "TIER" in the USA and "Stage" in Europe), aimed at the progressive reduction (through successive steps spaced out over time) of gaseous pollutant emissions produced by internal combustion engines, in relation to the maximum power of the engine and the sector of application. Attention was focused on four substances, namely carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides (NOx) and particulate matter (PM). For diesel engines, it was especially important to contain NOx and PM emissions. To reduce gaseous pollutants, action was taken both by limiting their formation in the combustion chamber and, above all, by inserting various types of filters downstream of the engine to physically block them and/or catalysts to render them chemically harmless. In fact, recently both strategies have been increasingly used. It is significant to underline that the Stage V (Tier 5) level, now required for engines installed on agricultural machinery, has reduced the environmental impact of the pollutants mentioned by up to an incredible 98% compared to Stage I engines, the limit in force initially. Technical methods for reducing pollutants EGR (Exhaust Gas Recirculation) Exhaust gas recirculation (EGR) is a solution based on the reintroduction of a portion of the exhaust gas (approximately 515% in volume) into the combustion chamber, thereby lowering the combustion temperature and consequently reducing the production of NOx. This process is usually carried out externally, with the interception of a variable part of the exhaust gases by means of a special electronically-controlled valve. The gases are then cooled through a heat exchanger to reduce their temperature and increase their density. They are then re-introduced into the combustion chamber together with the diesel fuel and combustion air. SCR (Selective Catalytic Reduction) This method makes it possible to effectively reduce the NOx level through the subsequent treatment of the exhaust gases after they are generated, and thus without interfering with combustion. Inside a catalytic converter, and in a manner controlled by a special electronic control unit, a 32.5% pure aqueous solution of urea (often commercially referred to as "AdBlue") is vaporized and placed into the exhaust gas flow. At high temperatures, it dissociates, releasing ammonia. The latter, together with oxygen, combines in the catalyst with nitrogen oxides, giving off atmospheric nitrogen (non-toxic) and water, in the form of steam. DPF (Diesel Particulate Filter) This is a physical filter made of extremely selective refractory material, which manages to trap larger particles on the typical section of its lattice. Periodic cleaning of the DPF (called “regeneration”) can be passive, occurring when the temperature of the exhaust gases is particularly high, or active, with the modification of the combustion of the diesel fuel, in order to raise this temperature to trigger the combustion of the carbon particles trapped in the filter with a corresponding reduction in terms of average size and their consequent expulsion. DOC (Diesel Oxidation Catalyst) The diesel oxidation catalyst (DOC) acts on unburned hydrocarbons (HC) and carbon monoxide (CO), which are converted into CO2 and water vapor. DOC is made up of a honeycomb ceramic matrix, upon which a thin layer of noble metals (platinum, palladium, rhodium) is deposited, which act as catalysts to facilitate the appropriate chemical reactions in an accelerated manner. Strategies and devices to reduce gaseous pollutant emissions dosing module modulo di alimentazione power supply module serbatoio/ tank idrocarburi incombusti unburned Hydrocarbons monossido di carbonio carbon monoxide vapore acqueo water vapor particolato particulate frazione organica solubile soluble organic fraction anidride carbonica carbon dioxide exhaust gas with NOx catalizzatore catalytic SCR azoto e vapore acqueo nitrogen and water vapor
RkJQdWJsaXNoZXIy NTY4ODI=